<style>
   .colorBox {
    width: 200px;
    height: 200px;
    border: 1px solid #ccc;
  }
</style>

<div id="app">
  <div class="colorBox" :style="`backgroundColor:rgb(${r},${g},${b})`"></div>
  <div>
    R:<input type="range" v-model="r" max="250"> {{r}}
    G:<input type="range" v-model="g" max="250"> {{g}}
    B:<input type="range" v-model="b" max="250"> {{b}}
  </div>
</div>
<script src="https://cdn.jsdelivr.net/npm/vue"></script>
<script>
  const vm = new Vue({
    el: '#app',
    data: {
      r: 0,
      g: 0,
      b: 0
    }
  })
</script>